In lifting mechanisms and buckets having two scoops which are pivotally moveable to pick up a load, separate hoists are commonly used for lifting and lowering the bucket and for opening and closing the bucket scoops. The hoist used to lift and lower the bucket is generally referred to as the hold hoist and the hoist used to open and close the bucket scoops is generally known as the close hoist. Due to the use of the two hoists, such bucket apparatus are commonly known as double hoist buckets. Although the primary use of the bucket opening and closing hoist and the ropes which extend from it to the bucket is to operate the two bucket scoops, because two hoists are required for the bucket scoop operation and the lifting of the bucket, it is also desirable and common to use both hoists for lifting the bucket and material which it carries and to share the total load as equally as possible between the two hoists. Although the foregoing and ongoing are with reference to buckets having scoops for carrying material, they also apply to grapples having hooks for carrying material.
A common type of drive for both the close and hold hoists is an alternating current wound rotor induction motor for each hoist. The wound rotor motor for the close hoist can be operated separately to open and close the bucket and the wound rotor motor for the hold hoist can be used separately to raise or lower the bucket. The two motors can be used together to raise or lower the bucket and can also be used in conjunction with each other to provide bucket opening and closing manipulation while also raising or lowering the bucket. The use of wound rotor motors having external resistors which can be switched into and out of each motor rotor circuit and a braking system using, e.g., eddy current brakes, provides speed matching and flexibility from speed to torque curves which have their slope varied by the insertion or removal of the resistance and use of the brakes.
However, wound rotor motor drives have well-known drawbacks which include the high cost of such motors, brakes and the external resistors, high maintenance, poor efficiency, and problems relating to dissipation of the heat generated by the external resistors. In the instant invention, squirrel cage a.c. induction motors or relatively inexpensive wound rotor motors are used and the problems of the typical wound rotor motor drives previously used are eliminated.